1. Field of the Invention
This invention relates to a magnetoresistance effect element using a current perpendicular-to-the-plane (CPP) system, a magnetic head including the magnetoresistance effect element, a head suspension assembly, and a magnetic reproducing apparatus. In the CPP system, a sensing current flows perpendicular to the direction in which a plurality of conductive layers are stacked.
2. Description of the Related Art
In recent years, the size of magnetic recording apparatuses, including hard disk units, has been rapidly getting smaller and so the recording density has been getting higher. This trend is expected to become stronger in the future. To achieve high recording density, it is necessary not only to increase the recording track density by narrowing the recording tracks but also to increase the recoding density (or line recording density) in which recording is done.
To reproduce the signal recorded on a medium, an induction head has been used in the existing techniques. As the recording density gets higher, the recording track width becomes narrower. Consequently, the recording bit size gets smaller, with the result that a sufficient reproduced signal output cannot be obtained with the induction head. To overcome this drawback, head using an anisotropic magnetoresistance (AMR) effect (AMR) has been developed. Such an AMR head has been provided under the name of a shield reproduction head. Recently, by making use of the giant magnetoresistance (GMR) effect, a spin valve GMR head with much higher sensitivity has been used. Use of these reproduction heads enables a reproduced signal output of a sufficient level to be obtained, even when the recording bit size is small.
The development of a magnetic head using a tunnel magnetoresistance (TMR) effect element or a CPP-GMR element is in progress and the way of putting the magnetic head into practical use is under investigation. In the existing current-in-plane giant magnetoresistance (CIP-GMR) element, a sensing current flows in the surface of the conductive film. In contrast, in a TMR element or a CPP-GMR element, a sensing current flows in a direction perpendicular to the surface of the conductive film.
A CPP-GMR element has been disclosed in, for example, Jpn. Pat. Appln. KOKAI 10-55512 (reference 1) and U.S. Pat. No. 5,668,688 (reference 2). As disclosed in these references, magnetic heads with a high reproducing sensitivity have been developed. Use of them enables the recorded signal to be reproduced, even when the recording bit size becomes smaller.
It is known that, in the CPP-GMR element, since the resistance of the CPP-GMR film is small in the direction of the film thickness, the absolute value of the amount of resistance change is small and therefore a high output is difficult to obtain. In this connection, a CPP-GMR element which uses a current confining effect to realize a suitable resistance and a high rate of resistance change has been disclosed (e.g., refer to Jpn. Pat. Appln. KOKAI 2002-208744 (reference 3) or U.S. Pat. No. 6,560,077 (reference 4)). The current confining effect is to cause current to flow in the conducting parts scattered in a layer composed mainly of an insulating material in such a manner that the current narrows, thereby increase the rate of resistance change. Hereinafter, a layer which produces a current confining effect is referred to as a current control layer. Reference 3 has disclosed a magnetoresistance effect element which has a plurality of current control layers in a single unit composed of a plurality of conductive layers.
When a plurality of current control layers are used, the positions of the conductive parts of the individual current control layers are very significant. As written in [0078] in reference 3, when the positions of the conductive parts (pinholes) of a plurality of current control layers differ from one another, the resistance value itself can be increased. However, recent research has shown that the current confining effect weakens the effect of increasing the rate of resistance change. Since it is difficult to align the positions of the conductive parts of a plurality of current control layers by the existing techniques, some suitable measures should be taken.
In addition, since there is a limit to the thickness of the current control layers, the application of a voltage higher than the breakdown voltage to the insulating material causes dielectric breakdown, or breakdown. This means that there is a limit to a sensing current that can be applied and therefore the output of the element reaches the highest limit. Since the breakdown is one factor which causes deterioration with age, it decreases the long-term reliability of the magnetoresistance effect element.